Color display device

ABSTRACT

The present invention provides a reflective color display device which can display multiple color states, without the disadvantages associated with previously known color display devices. The display fluid of the present invention comprises (a) black and white electrophoretic particles which are oppositely charged and (b) charged color-generating particles having photonic crystal characteristics, all of which are dispersed in a solvent or solvent mixture.

This application claims the benefit of U.S. Provisional Application 61/955,129, filed Mar. 18, 2014, which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention is directed to a color display device which can display high quality color states, and a display fluid for such a color display device.

BACKGROUND OF THE INVENTION

In order to achieve a color display, color filters are often used. The most common approach is to add color filters on top of black/white sub-pixels of a pixellated display to display the red, green and blue colors. When a red color is desired, the green and blue sub-pixels are turned to the black state so that the only color displayed is red. When the black state is desired, all three-sub-pixels are turned to the black state. When the white state is desired, the three sub-pixels are turned to red, green and blue, respectively, and as a result, a white state is seen by the viewer.

The biggest disadvantage of such a technique is that since each of the sub-pixels has a reflectance of about one third (⅓) of the desired white state, the white state is fairly dim. To compensate this, a fourth sub-pixel may be added which can display only the black and white states, so that the white level is doubled at the expense of the red, green or blue color level (where each sub-pixel is only one fourth [¼] of the area of the pixel). Brighter colors can be achieved by adding light from the white pixel, but this is achieved at the expense of color gamut to cause the colors to be very light and unsaturated. A similar result can be achieved by reducing the color saturation of the three sub-pixels. Even with this approach, the white level is normally substantially less than half of that of a black and white display, rendering it an unacceptable choice for display devices, such as e-readers or displays that need well readable black-white brightness and contrast.

An alternative of color display device employs colored pigment particles, in addition to the black and white particles. This type of color display can display multiple color states by moving the black, white and colored particles to the viewing side. However, the number of the color states displayed is limited by how many types of different colored particles are in the display fluid and how well their movement can be controlled.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a display fluid of the present invention.

FIGS. 2-1 to 2-5 illustrate how different color states may be displayed by the color display device of the present invention.

FIG. 3 demonstrates how locations of the black and white particles influence the intensity/brightness of the color displayed by the color-generating particles.

FIG. 4 demonstrates how mixing levels of the black and white particles influence the intensity/brightness of the color displayed by the color-generating particles.

SUMMARY OF THE INVENTION

The present invention is directed to a display fluid comprising

(a) black and white electrophoretic particles which are oppositely charged, and

(b) charged color-generating particles having photonic crystal characteristics, all of which are dispersed in a solvent or solvent mixture.

In one embodiment, all of the color-generating particles are either positively or negatively charged.

In one embodiment, the charged color-generating particles have electrical polarization characteristics. In one embodiment, the solvent or solvent mixture has electrical polarization characteristics. In one embodiment, both the color-generating particles and the solvent or solvent mixture have electrical polarization characteristics.

In one embodiment, the color-generating particles are formed of silicon (Si), titanium (Ti), barium (Ba), strontium (Sr), iron (Fe), nickel (Ni), cobalt (Co), lead (Pb), aluminum (Al), copper (Cu), silver (Ag), gold (Au), tungsten (W), molybdenum (Mo), or a compound thereof.

In one embodiment, the color-generating particles are formed of polymer materials such as PS (polystyrene), PE (polyethylene), PP (polypropylene), PVC (polyvinyl chloride), or PET (polyethylene terephthalate).

In one embodiment, the color-generating particles are formed by coating particles or a cluster having no electric charge with a material having electric charges.

In one embodiment, the color-generating particles include a material which is electrically polarized with any one of electronic polarization, ionic polarization, interfacial polarization or rotational polarization due to asymmetrical charge distribution of atoms or molecules as an external electric field is applied.

In one embodiment, the color-generating particles include a ferroelectric material.

In one embodiment, the color-generating particles include a superparaelectric material.

In one embodiment, the color-generating particles include a material having a perovskite structure.

In one embodiment, the solvent is water, trichloroethylene, carbon tetrachloride, di-iso-propyl ether, toluene, methyl-t-bytyl ether, xylene, benzene, diethyl ether, dichloromethane, 1,2-dichloroethane, butyl acetate, iso-propanol, n-butanol, tetrahydrofuran, n-propanol, chloroform, ethyl acetate, 2-butanone, dioxane, acetone, methanol, ethanol, acetonitrile, acetic acid, dimethylformamide, dimethyl sulfoxide or propylene carbonate.

In one embodiment, the present invention is directed to a method for generating a full spectrum of colors, which method comprises applying an electric field to the display fluid of the present invention to control the inter-particle distances of the color-generating particles.

In one embodiment, the intensity of the colors displayed is controlled by adjusting locations of the black and white electrophoretic particles or mixing levels of the black and white particles.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a reflective color display device which can display multiple color states, without the disadvantages associated with previously known color display devices.

The display fluid of the present invention, as shown in FIG. 1, comprises (a) black (11) and white (12) electrophoretic particles which are oppositely charged and (b) charged color-generating particles (13) having photonic crystal characteristics, all of which are dispersed in a solvent or solvent mixture. All the color-generating particles carry the same charge polarity, positive or negative.

The display fluid is sandwiched between two electrode layers. One of the electrode layers is a common electrode (14) which is a transparent electrode layer (e.g., ITO), spreading over the entire top of the display device. The other electrode layer (15) is a layer of pixel electrodes (15 a).

The pixel electrodes are described in U.S. Pat. No. 7,046,228, the content of which is incorporated herein by reference in its entirety. It is noted that while active matrix driving with a thin film transistor (TFT) backplane is mentioned for the layer of pixel electrodes, the scope of the present invention encompasses other types of electrode addressing as long as the electrodes serve the desired functions.

The black electrophoretic particles (11) may be formed from CI pigment black 26 or 28 or the like (e.g., manganese ferrite black spinel or copper chromite black spinel) or carbon black.

The white electrophoretic particles (12) may be formed from an inorganic pigment, such as TiO₂, ZrO₂, ZnO, Al₂O₃, Sb₂O₃, BaSO₄, PbSO₄ or the like.

As stated, the black and white particles are oppositely charged. If the black particles are positively charged, then the white particles are negatively charged, or vice versa.

The percentages of the black and white particles in the fluid may vary. For example, the black electrophoretic particle may take up 0.1% to 10%, preferably 0.5% to 5%, by volume of the electrophoretic fluid; the white electrophoretic particle may take up 1% to 50%, preferably 5% to 20%, by volume of the fluid.

The size of the black and white particles in the fluid may vary. For example, both the black and white particle may have a size between 100 nm to 10 um, preferably between 200 nm to 1 um.

The charged color generating particles having photonic crystal characteristics are described in U.S. Pat. No. 8,238,022. Some of the description in U.S. Pat. No. 8,238,022 is quoted below. However, it is noted that the content of the entire patent is incorporated herein by reference.

The color generating particles are charged. They may have electrical polarization characteristics or the solvent may have electrical polarization characteristics or both may have electrical polarization characteristics. In any case, the inter-particle distances may be controlled by applying an electric field to the display fluid, thereby implementing a full spectrum of colors using the photonic crystal characteristics of the color-generating particles.

All of the color-generating particles carry the same charge polarity, either positive or negative. They may be arranged at predetermined spaces from each other by the repulsive force between them caused by electric charges of the same polarity.

The diameter of the color-generating particles may range from several nm to several hundred microns; but the particle diameter is not necessarily limited thereto.

As indicated in U.S. Pat. No. 8,238,022, the color-generating particles may be formed of elements, such as silicon (Si), titanium (Ti), barium (Ba), strontium (Sr), iron (Fe), nickel (Ni), cobalt (Co), lead (Pb), aluminum (Al), copper (Cu), silver (Ag), gold (Au), tungsten (W), molybdenum (Mo), or a compound thereof. In addition, the color-generating particles may be formed of polymer materials such as PS (polystyrene), PE (polyethylene), PP (polypropylene), PVC (polyvinyl chloride), or PET (polyethylene terephthalate).

Furthermore, the color-generating particles may be formed by coating particles or a cluster having no electric charge with a material having electric charges. Examples of these particles may include particles whose surfaces are processed (or coated) with an organic compound having a hydrocarbon group; particles whose surfaces are processed (or coated) with an organic compound having a carboxylic acid group, an ester group or an acyl group; particles whose surfaces are processed (or coated) with a complex compound containing halogen (F, CI, Br or I) elements; particles whose surfaces are processed (or coated) with a coordination compound containing amine, thiol or phosphine; and particles having electric charges generated by forming radicals on the surfaces.

Meanwhile, in order for the color-generating particles to effectively exhibit photonic crystal characteristics by maintaining a stable colloidal state without precipitation in a solvent, the value of the electrokinetic potential (i.e., zeta potential) of the colloidal solution (comprising the particles and a solvent) may be greater than or equal to a preset value. For example, the absolute value of the electrokinetic potential of the colloidal solution may be more than or equal to 10 mV. In addition, the difference in specific gravity between the particles and the solvent may be less than or equal to a preset value, for example, less than or equal to 5. Furthermore, the difference in refractive index between the solvent and the particles may be greater than or equal to a preset value, for example, more than or equal to 0.3.

Further, if the color-generating particles have electrical polarization characteristics, the particles may include a material which is electrically polarized with any one of electronic polarization, ionic polarization, interfacial polarization or rotational polarization due to asymmetrical charge distribution of atoms or molecules as an external electric field is applied.

Moreover, the color-generating particles may include a ferroelectric material, which shows an increase in polarization upon application of an external electric field and shows a large remnant polarization and remnant hysteresis even without the application of an external electric field. Alternatively, the color-generating particles may include a superparaelectric material, which shows an increase in polarization upon application of an external electric field and shows no remnant polarization and no remnant hysteresis when no external electric field is applied.

Further, the color-generating particles may include a material having a perovskite structure. Examples of materials having a perovskite structure, such as ABO₃, may include materials such as PbZrO₃, PbTiO₃, Pb(Zr,Ti)O₃, SrTiO₃ BaTiO₃, (Ba, Sr)TiO₃, CaTiO₃, LiNbO₃ or the like.

If the color-generating particles have electrical polarization characteristics, the solvent does not have to have electrical polarization characteristics. In this case, the solvent may be a dielectric solvent, examples of which include, but are not limited to, solvents having a low viscosity and a dielectric constant in the range of about 2 to about 30, preferably about 2 to about 15 for high particle mobility. Specific examples of suitable dielectric solvent may include hydrocarbons such as isopar, decahydronaphthalene (DECALIN), 5-ethylidene-2-norbornene, fatty oils, paraffin oil; silicon fluids; aromatic hydrocarbons such as toluene, xylene, phenylxylylethane, dodecylbenzene and alkylnaphthalene; halogenated solvents such as perfluorodecalin, perfluorotoluene, perfluoroxylene, dichlorobenzotrifluoride, 3,4,5-trichlorobenzotrifluoride, chloropentafluoro-benzene, dichlorononane, pentachlorobenzene; and perfluorinated solvents such as FC-43, FC-70 and FC-5060 from 3M Company, St. Paul Minn., low molecular weight halogen containing polymers such as poly(perfluoropropylene oxide) from TCI America, Portland, Oreg., poly(chlorotrifluoro-ethylene) such as Halocarbon Oils from Halocarbon Product Corp., River Edge, N.J., perfluoropolyalkylether such as Galden from Ausimont or Krytox Oils and Greases K-Fluid Series from DuPont, Del., polydimethylsiloxane based silicone oil from Dow-corning (DC-200).

If the color-generating particles do not have electrical polarization characteristics, the solvent has to have electrical polarization characteristics, which may be created by methods/materials as described above for the color-generating particles. For example, the solvent may include a material which is electrically polarized with any one of electronic polarization, ionic polarization, interfacial polarization, or rotational polarization due to asymmetrical charge distribution of atoms or molecules as an external electric field is applied; or the solvent may include a ferroelectric material; or the solvent may include a superparaelectric material; or the solvent may include a material having a perovskite structure as described above; or the solvent may include a material having a polarity index of 1 or greater.

Examples of solvent having electrical polarization characteristics may include, but are not limited to, water, trichloroethylene, carbon tetrachloride, di-iso-propyl ether, toluene, methyl-t-bytyl ether, xylene, benzene, diethyl ether, dichloromethane, 1,2-dichloroethane, butyl acetate, iso-propanol, n-butanol, tetrahydrofuran, n-propanol, chloroform, ethyl acetate, 2-butanone, dioxane, acetone, methanol, ethanol, acetonitrile, acetic acid, dimethylformamide, dimethyl sulfoxide and propylene carbonate.

The color-generating particles carrying the same charge polarity are dispersed in a solvent which has electrical polarization characteristics. When an electric field is applied to the dispersion, electrical attraction proportional to the intensity of the electric field and the charge amount of the particles, act on the particles due to the electric charges of the particles. As a result, the particles move in a predetermined direction by electrophoresis, thus narrowing the inter-particle distances. In contrast, electrical repulsion generated between the particles having the electric charges of the same polarity increases as the inter-particle distances become smaller resulting in a predetermined equilibrium state while preventing the inter-particle distances from continuing to decrease.

Further, the solvent is electrically polarized in a predetermined direction due to the electrical polarization characteristics of the solvent. Thus, electrical attraction is locally generated and exerts a predetermined effect upon the inter-particle distances between the particles electrically interacting with the polarized solvent. That is, the color-generating particles can be regularly arranged at distances where electrical attraction induced by an external electric field, electrical repulsion between the particles having electric charges of the same polarity and electrical attraction induced by polarization, are in equilibrium. As a result, the inter-particle distances can be controlled at predetermined levels, and the particles arranged at predetermined distances can function as photonic crystals. Since the wavelength of light reflected from the regularly spaced particles is determined by the inter-particle distance, the wavelength of the light reflected from the particles can be arbitrarily controlled by controlling the inter-particle distances. Therefore, a pattern of the wavelength of reflected light may be diversely represented by the factors, such as the intensity and direction of the applied electric field, the size and mass of the particles, the refractive indices of the particles and the solvent, the charge amount of the particles, the electrical polarization characteristics of the solvent or the concentration of the particles dispersed in the solvent.

Alternatively, when the color-generating particles having both electric charges of the same polarity and electrical polarization characteristics are dispersed in a solvent and if an electric field is applied to the particles and the solvent, electrical attraction proportional to the intensity of the electric field and the charge amount of the particles act on the particles due to the electric charges of the particles. Therefore, the particles move in a predetermined direction by electrophoresis, thus narrowing the inter-particle distance. In contrast, electrical repulsion generated between the particles having the electric charges of the same polarity increases as the inter-particle distances decreases, thus reaching a predetermined equilibrium state while preventing the inter-particle distances from continuing to decrease. The particles are electrically polarized in a predetermined direction due to the electrical polarization characteristics of the particles. Thus, electrical attraction is locally generated between the polarized particles and exerts a predetermined effect upon the inter-particle distances.

As a result, the color-generating particles can be regularly arranged at a distance where electrical attraction induced by an external electric field, electrical repulsion between the particles having electric charges of the same polarity and electrical attraction induced by polarization, are in equilibrium. Accordingly, the inter-particle distances can be controlled at predetermined intervals, and the particles arranged at predetermined intervals can function as photonic crystals. Since the wavelength of light reflected from the regularly arranged color-generating particles is determined by the inter-particle distances, the wavelength of the light reflected from the particles can be accurately controlled by controlling the inter-particle distances. Therefore, a pattern of the wavelength of reflected light may be diversely represented by the factors, such as the intensity and direction of an electric field, the size and mass of the particles, the refractive indices of the particles and the solvent, the charge amount of the particles, the electrical polarization characteristics of the particles or the concentration of the particles dispersed in the solvent.

It is possible for both the color-generating particles and the solvent to have electrical polarization characteristics.

FIGS. 2-1 to 2-5 illustrate how different color states may be displayed by a display device of the present invention.

As shown, a display fluid comprises two types of electrophoretic particles, black (21) and white (22), and one type of color-generating particles (23). It is assumed that the black electrophoretic particles are positively charged and the white electrophoretic particles are negatively charged. The color-generating particles carry a positive charge and the charge level of the color-generating particles is lower than that of the charges carried by the black and white particles.

When a high positive driving voltage V2 is applied for a short period of time t2, the positively charged black particles are driven to the viewing side (i.e., the side of the common electrode). As a result, a black color is seen (FIG. 2 a). The high driving voltage, in this case, is referred to as a driving voltage which is sufficiently high to drive the black particles to the viewing side during a short driving time (i.e., t2). Such a driving voltage may be +15V, as an example. The short driving time t2 is usually less than 500 msec.

When a high negative driving voltage V1 is applied for a short period of time t1, the negatively charged white particles are driven to the viewing side. As a result, a white color is seen (FIG. 2 b). The high driving voltage, in this case, is referred to as a driving voltage which is sufficiently high to drive the white particles to the viewing side during a short driving time, t1. Such a driving voltage may be −15V, as an example. The short driving time is usually also less than 500 msec.

In FIGS. 2 a and 2 b, because the color-generating particles are lesser charged and the driving times are short, the color-generating particles remain scattered in the display fluid.

If a negative driving voltage V3 is applied to the fluid in FIG. 2 a for a period of time, t3, which driving voltage and the driving time are not sufficient to drive a pixel to the white color state of FIG. 2 b, and instead, the white and black particles are driven to the middle of the pixel as shown in FIG. 2 c, the color seen would be the color (i.e., a first color state) of the color-generating particles.

Similarly, if a positive driving voltage V4 is applied to the fluid in FIG. 2 b for a period of time, t4, which driving voltage and the driving time are not sufficient to drive a pixel to the black color state of FIG. 2 a, and instead, the white and black particles are driven to gather in the middle of the pixel as shown in FIG. 2 d, the color seen would also be the color of the color-generating particles.

The brightness of the color of the color-generating particles can be adjusted by controlling locations of the black and white particles and mixing levels through magnitude of driving voltages V3 and V4 and/or driving times, t3 and t4. The magnitude of driving voltages V3 and V4, in this case, may be higher than, or equal to, 10V. The driving times, t3 and t4, may be the same as t1 and t2, but in this case, they are preferably shorter than t1 and t2.

When a low positive driving voltage V5 is applied to the fluid in FIG. 2 c and the voltage is applied for a relatively long period of time, t5, an external electric field created between the common electrode and the pixel electrode would alter the spatial distances between the color-generating particles. As a result, the color-generating particles would reflect light of a different wavelength (i.e., a second color state of FIG. 2 e), according to the applied voltage. The color or the reflective light spectrum from color-generating particles is adjustable by driving voltage V5 and it continuously changes with the change of the driving voltage. Usually with higher driving voltage, the spectrum of reflective light shifts from low frequency to high frequency range. The reflective light band may cover not only the range of visible light but also the infrared and ultraviolet light ranges. The low driving voltage V5, in this case, is usually lower than V1 and V2, because of which the black and white particles in this scenario remain substantially unmoved. V5 may be less than, or equal to, +5V, as an example. The time period, t5, is usually longer than t1 and t2.

The same phenomenon may also be achieved with V6 applied to the fluid of FIG. 2 c for a relatively long period of time, t6, to generate a third color state of a different wavelength (see FIG. 2 f).

When a voltage of V7 is applied to the fluid of FIG. 2 e for a period of time, t7, a third color state may be displayed (see FIG. 2 f).

The driving voltages V6 and V7 are lower than V1 and V2 and the time periods t6 and t7 are longer than t1 and t2.

The phenomenon illustrated in FIG. 2 for the fluid of (c), (e) and (f) can be similarly applied to the fluid of (d), (e) and (f), with driving voltages, V8, V9 and V10 and driving times, t8, t9 and t10, respectively. For example, V8, V9 and V10 are usually lower than V1 and V2 and the time periods t8, t9 and t10 are longer than t1 and t2. The driving voltages and driving times, V5-V7 and t5-t7, may be the same or different from the driving voltages and driving times, V8-V10 and t8-t10.

The colors referred to in the drawings, a first color, a second color and a third color, may be red, green and blue, respectively. However, this is in no way limiting the scope of the present invention. It is noted that each of the pixels in the present color display device may display an unlimited number of color states, because when different external electric fields are applied, they would cause the color-generating particles to reflect light of different wavelengths.

As stated above, the brightness of the color of the color-generating particles can be adjusted by controlling the locations and mixing levels of the black and white particles. This is shown in FIGS. 3 and 4, respectively.

In FIG. 3, pixel (a) has the highest brightness of the color generated by the color-generating particles as the white particles are on top of the black particles and are the closest to the viewing side. Pixels (b) and (c) have brightness lower than pixel (a). Pixel (d) is darker than pixel (c) as in pixel (d), the black particles are on top of the white particles. Pixel (f) has the lowest brightness because the black particles are closest to the viewing side.

FIG. 4 shows how different mixing levels of the black and white particles influence the brightness of the color generated by the color-generating particles. In pixel (a), there are more black particles than the white particles facing the viewing side and therefore that pixel is darker than the other pixels. In pixel (c), there are more white particles than the black particles facing the viewing side and therefore that pixel is brighter. In pixel (b), the black and white particles are more evenly distributed which causes the color intensity between pixel (a) and pixel (c).

While the present invention has been described with reference to the specific embodiments thereof, it should be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation, materials, compositions, processes, process step or steps, to the objective and scope of the present invention. All such modifications are intended to be within the scope of the claims appended hereto. 

What is claimed is:
 1. A display fluid comprising: (a) black and white electrophoretic particles which are oppositely charged, and (b) charged color-generating particles having photonic crystal characteristics; all of which are dispersed in a solvent or solvent mixture.
 2. The fluid of claim 1, wherein all of the color-generating particles are either positively or negatively charged.
 3. The fluid of claim 1, wherein the charged color-generating particles have electrical polarization characteristics.
 4. The fluid of claim 1, wherein the solvent or solvent mixture has electrical polarization characteristics.
 5. The fluid of claim 1, wherein both the color-generating particles and the solvent or solvent mixture have electrical polarization characteristics.
 6. The fluid of claim 1, wherein the color-generating particles are formed of silicon (Si), titanium (Ti), barium (Ba), strontium (Sr), iron (Fe), nickel (Ni), cobalt (Co), lead (Pb), aluminum (Al), copper (Cu), silver (Ag), gold (Au), tungsten (W), molybdenum (Mo), or a compound thereof.
 7. The fluid of claim 1, wherein the color-generating particles are formed of polymer materials such as PS (polystyrene), PE (polyethylene), PP (polypropylene), PVC (polyvinyl chloride), or PET (polyethylene terephthalate).
 8. The fluid of claim 1, wherein the color-generating particles are formed by coating particles or a cluster having no electric charge with a material having electric charges.
 9. The fluid of claim 3, the color-generating particles include a material which is electrically polarized with any one of electronic polarization, ionic polarization, interfacial polarization or rotational polarization due to asymmetrical charge distribution of atoms or molecules as an external electric field is applied.
 10. The fluid of claim 1, wherein the color-generating particles include a ferroelectric material.
 11. The fluid of claim 1, wherein the color-generating particles include a superparaelectric material.
 12. The fluid of claim 1, wherein the color-generating particles include a material having a perovskite structure.
 13. The fluid of claim 4, wherein the solvent is water, trichloroethylene, carbon tetrachloride, di-iso-propyl ether, toluene, methyl-t-bytyl ether, xylene, benzene, diethyl ether, dichloromethane, 1,2-dichloroethane, butyl acetate, iso-propanol, n-butanol, tetrahydrofuran, n-propanol, chloroform, ethyl acetate, 2-butanone, dioxane, acetone, methanol, ethanol, acetonitrile, acetic acid, dimethylformamide, dimethyl sulfoxide or propylene carbonate.
 14. A method for generating a full spectrum of colors, comprising applying an electric field to the display fluid of claim 1 to control the inter-particle distances of the color-generating particles.
 15. The method of claim 14, wherein the intensity of the colors displayed is controlled by adjusting locations of the black and white electrophoretic particles or mixing levels of the black and white particles. 